Cathode Bias Red Plating Discussion

[Mr_bibbles]

I thought it might be another flyback, but it's not connected to cathode. Also, I don't even know if you can use a flyback for cathode bias.

The anode goes to x2 paralleled 1k resistors, which goes to the grid. The cathode is tied to the legs of two resistors, one 24k and one 10k, but I cannot find where the path goes after the resistors. Both are close to what I assume are the Coupling capacitors for the PI.... part of a long tail inverter???

Note- this Diode DOES measure well in/out of circuit. Numbers are a bit scratched from removing it, but looks like 400-X- . In4007?


Any thoughts on the other IN4007 tied to cathode? Considering that removing it stopped the red plating but otherwise sounded passable at the very least, that seems to be a prime suspect?

[Chuck H]

I'm afraid a schematic is in order as it seems there is an unusual circuit here. Trying to analyze it based on typical circuits (ie: Rk, Ck) is useless now and, I'm sorry, but the diode implementation is unclear.

A diode with it's cathode to ground parallel to the cathode resistor pretty much moots the cathode resistance and the power tube cathodes are grounded. Without a negative voltage on the grids there is almost no bias. Ergo, red plating. The amp wouldn't have been designed like this.

Maybe it would help to know some back story on this amp.?. Do you know if it ever worked? Do you know more certainly that another person has been modifying it or trying to "fix" it? It's a contemporary design, was it beyond warranty when it failed initially? Did someone void the warranty by poking around in it or is it possibly stolen ?

Working from inside a box (knowledge of typical designs) trying to diagnose outside the box while waiting for Easter eggs of circuit info is becoming frustrating.

[Mr_bibbles]

I'm afraid a schematic is in order as it seems there is an unusual circuit here. Trying to analyze it based on typical circuits (ie: Rk, Ck) is useless now and, I'm sorry, but the diode implementation is unclear.

A diode with it's cathode to ground parallel to the cathode resistor pretty much moots the cathode resistance and the power tube cathodes are grounded. Without a negative voltage on the grids there is almost no bias. Ergo, red plating. The amp wouldn't have been designed like this.

Maybe it would help to know some back story on this amp.?. Do you know if it ever worked? Do you know more certainly that another person has been modifying it or trying to "fix" it? It's a contemporary design, was it beyond warranty when it failed initially? Did someone void the warranty by poking around in it or is it possibly stolen ?

Working from inside a box (knowledge of typical designs) trying to diagnose outside the box while waiting for Easter eggs of circuit info is becoming frustrating.

Haha, here's the backstory. Not stolen, someone brought it to me after having it worked on at another shop. They had a bad experience, couldn't get it fixed there, and I'm guessing it's because there's no schematic available for these and it's a bit of a funkier problem.

Called Tony from Krank, he was not able to help besides telling me "it's cathode biased", and no schematics exist of this online. Another caveat is that the board cannot be removed unless you unsolder the tube sockets from the board- they were soldered in after the board was set in place. I think unsoldering them is my next move if we cannot determine what this diode is. Everything looks original from the outset, but it's possible another tech lifter / added a parallel cathode resistor. because it's wired directly from pin 1 to a connector pin tied to ground.

Chuck, I think I understand what you mean about the diode- if it's parallel to the resistor, current would flow with no resistance to ground, so no tech in their right mind would ever use one here. Perhaps there is a failed cap between cathode and this diode?? Or, as G1 astutely guessed a while ago, a board short.

Assuming it's SUPPOSED to be there, would anyone else have ideas about what the purpose of the diode to ground is?

[Chuck H]

Here's a question, that diode that seems to be across the cathode resistor has it's cathode terminated at ground, so... I'm sure you tested that termination as ground, but is it supposed to be? Is it possible that diodes cathode terminates on a trace that shouldn't be ground? You can probably determine this by looking at other circuits connected to that trace. If it seems the trace shouldn't be grounded then you can look for the board short that g1 suggested or maybe another shorted component.

[g1]

Agree with Chuck and was also going to suggest you verify the diode connections by following the traces if you have not done that.
If you were just going by resistance readings, it's possible there is something shorted between tube cathode and diode anode. Or something shorted between diode cathode and ground. Trace it out visually.
Also, does it look stock or like someone may have added it in?

[Mr_bibbles]

Agree with Chuck and was also going to suggest you verify the diode connections by following the traces if you have not done that.
If you were just going by resistance readings, it's possible there is something shorted between tube cathode and diode anode. Or something shorted between diode cathode and ground. Trace it out visually.
Also, does it look stock or like someone may have added it in?

Here's a question, that diode that seems to be across the cathode resistor has it's cathode terminated at ground, so... I'm sure you tested that termination as ground, but is it supposed to be? Is it possible that diodes cathode terminates on a trace that shouldn't be ground? You can probably determine this by looking at other circuits connected to that trace. If it seems the trace shouldn't be grounded then you can look for the board short that g1 suggested or maybe another shorted component.

Thanks guys, unfortunately the traces are not visible, they're either under the board or "inside" the PCB like on SMD boards, I've tried tracing it with a meter but of course, with it being tied to ground.....

I think this is getting to the heart of the problem though, I'll be unsoldering the tube sockets and tearing it out of it's berth.... I'll let you know what I find in case anyone else has these issues.

Also, this diode is marked correctly on the board in print, factory installed.

[eschertron]

I've had iffy (read: better than nothing) results tracing a board by sticking a high-intensity diode on a wand under the board. It may reveal just enough shadowy information to suss out some of the unknown connections. If the board's not fiberglass/semi-opaque then this won't work so well.

[g1]

I've had iffy (read: better than nothing) results tracing a board by sticking a high-intensity diode on a wand under the board.

I was wondering what kind of diode divining rod you were building when it struck me you probably meant the light emitting variety.

[Enzo]

I had the same reaction... Some sort of sensing diode??? Then, oh... a light. I use a 100 watt bulb. I have a clamp on work light.

[eschertron]

Yes, typing past my bedtime leads to errors of omission. I'm thinking with the PCB still installed in the chassis, there might not be room to get a work light under the board.

[J M Fahey]

I was chiming in to say something and I see you are near, but not exactly there.

I think that "somebody" who must be an avid "Forum reader" or a Graduate at "You Tube University" has added the "Ruby Mod" or whatever they call that dreaded invention of biasing tubes with a Zener diode.

FWIW it might even come from Factory itself.

In any case, either Factory or user added Zener diode shorted, Service "Tech" thought "oh! ... a diode!!! I have a few of those!!!" and replaced it with 1 x 1N4007 ... or 2 in series, same thing ... and oriented so they can pass current ("why would the original one(s) be oriented the wrong way"?) [headscratch] ... the perfect Storm.

Diodes get forward biased under operation , drop only 1 or 2V, and give power tubes all the current they want, so they happily redplate *and* overload +B down so much that it about halves and in a short time will also burn the PT.

Of course Multimeter, set to a resistance reading scale, does NOT show any diode(s) there.
Out of curiosity, I would measure cathode to ground, on the diode scale.

An alternative theory (hey!! I saw it at Alex Jones Channel so it must be true ) could be that you have there a properly oriented (replacement) Zener, but way lower Zener voltage, so tubes are always underbiased (voltage wise) and over biased (current wise). Pick one.

I wonder why the mains fuse does not blow, although I would not be surprised at finding it "reinforced".

So I suggest first of all remove those damn cathode diodes and plase sensible value cathode resistors.

Amp "should" work as normal.

[J M Fahey]

Cathode resistor voltage drop (Measuring at both sides of resistor with tubes in) 1.985VDC

There are two Resistors wired in parallel, Possibly a modification, possibly intentional. There is no short from cathode to ground, but the tubes are still red-plating even with both resistors removed from the circuit.

If it flies like a diode, swims like a diode and quacks like a diode, then itīs a diode. (slightly adapted to current situation )

[Mr_bibbles]

I was chiming in to say something and I see you are near, but not exactly there.

I think that "somebody" who must be an avid "Forum reader" or a Graduate at "You Tube University" has added the "Ruby Mod" or whatever they call that dreaded invention of biasing tubes with a Zener diode.

FWIW it might even come from Factory itself.

In any case, either Factory or user added Zener diode shorted, Service "Tech" thought "oh! ... a diode!!! I have a few of those!!!" and replaced it with 1 x 1N4007 ... or 2 in series, same thing ... and oriented so they can pass current ("why would the original one(s) be oriented the wrong way"?) [headscratch] ... the perfect Storm.

Diodes get forward biased under operation , drop only 1 or 2V, and give power tubes all the current they want, so they happily redplate *and* overload +B down so much that it about halves and in a short time will also burn the PT.

Of course Multimeter, set to a resistance reading scale, does NOT show any diode(s) there.
Out of curiosity, I would measure cathode to ground, on the diode scale.

An alternative theory (hey!! I saw it at Alex Jones Channel so it must be true ) could be that you have there a properly oriented (replacement) Zener, but way lower Zener voltage, so tubes are always underbiased (voltage wise) and over biased (current wise). Pick one.

I wonder why the mains fuse does not blow, although I would not be surprised at finding it "reinforced".

So I suggest first of all remove those damn cathode diodes and plase sensible value cathode resistors.

Amp "should" work as normal.

Fahey! Thank you- This seems like the answer I was looking for. I have never heard of that diode implementation before but this would all make sense as to why it was working with the diode removed.

But I'm having a terrible day. One of those days where everything you touch turns to shit.

When I (re-)removed the diode and powered it up I magic smoked the 1k (grid?) resistors in parallel. I have no idea how, It could have been the tubes, left a snipped lead in, left it diconnected for testing and forgot to re-connect... I repaired the damage (and managed to shock the shit out of myself in the process... like I said, terrible day...) and fired it up on the limiter, worked fine, added different tubes, worked fine, not pulling too much current. BUT now there is no signal / hum / buzz. Traced a signal through to V1, noticed there is NO PLATE VOLTAGE on any preamp tubes, but there is 350 on the plates of the power section. Open dropping resistor? any other suggestions? I noticed there is only 2 diodes near the primaries for a rectifier... they measure OL / 0.000 on the diode tester, but if this IS the rectifier and it WAS shorted, wouldn't the DC be missing from the output section?

On the plus side, the red plating is gone.

[g1]

This should be unrelated. They would not have a separate rectifier for the preamp supply. Like you say, the power tubes are still getting plate voltage, so there must be a dropper resistor or fuse open somewhere.

[J M Fahey]

Agree.
Only you didnīt notice because bthe redplating tubes problem is way more dangerous and you (and probably an earlier Tech) focused on it only.
Hey, if at the same time a dog is biting your ankle and a mosquito is biting your ear ... what would be the focus of your attention?

I wouldnīt even be surprised that an earlier Tech disconnected the preamp trying to find what was pulling his +V down